The present invention relates to a fuel control method and a fuel control device of a gas turbine combustor for advanced humid air turbines for stably operating a low NOx gas turbine combustor mounted on an advanced humid air turbine.
Publicly known JP-A-2008-175098 discloses a technology relating to fuel control capable of maintaining flame stability while ensuring a low NOx performance of a combustor before and after starting of moisture addition in an advanced humid air turbine power-generating plant, in which an improvement is achieved in output and efficiency by adding a moisture to a gas turbine working fluid (air) to humidify the same and using the humidified air to recover thermal energy possessed by gas turbine exhaust gases.
Generally, due to changes in compressor intake air flow rate and a vibration characteristic of a rotating body upon an increase in number of revolutions at the start of a gas turbine, there is a tendency that a system is liable to become unstable due to disturbance as compared with the case after a rated revolution speed is reached. Also, in an advanced humid air turbine plant, disturbance is imparted to a gas turbine when addition of moisture is started in the course of an increase in number of revolutions, so that in order to ensure stability at the start, it is desirable to start addition of moisture in a state of partial loading after the rated revolution speed is reached.
On the other hand, in case of using a fuel, such as natural gas, kerosine, light gas oil, or the like, having a small nitrogen content, a major part of nitrogen oxides (NOx) generated in a gas turbine combustor is thermal NOx generated by oxidation of nitrogen contained in an air. Since generation of thermal NOx is high in temperature dependence, a decrease in flame temperature is generally a fundamental principle in a low NOx combustion method in gas turbines, which use these fuels.
As a measure for decrease in flame temperature, there is known premixed combustion, in which a fuel and an air is beforehand mixed and then burned. Also, in the case where an air for combustion is made high in temperature by a regenerator as in advanced humid air turbine plants, it is necessary to control a flame temperature appropriately to achieve a low NOx while preventing self-ignition of a fuel, and therefore a technology, in which a fuel and an air are jetted as a multiplicity of coaxial jets having a small diameter into a gas turbine combustion chamber and burned, is effective as disclosed in JP-A-2008-175098.
In such gas turbine combustors directed to a low NOx, in order to make a low NOx performance and flame stability compatible, it is essential to adjust a fuel-air ratio, which is a ratio of fuel flow rate and air flow rate, in a predetermined range.
Publicly known JP-A-07-189743 discloses, as a low NOx gas turbine combustor used in general gas turbines, a technology, in which a ratio of fuel flow and air flow supplied to a gas turbine combustor is regulated on the basis of that change in opening degree of a compressor inlet guide valve, which accompanies operation of a gas turbine, a change in atmospheric temperature, that change in air flow, which is attributable to a change in atmospheric pressure, and that change in fuel flow, which is attributable to a change in fuel temperature and fuel heating value.
When addition of moisture is started in an advanced humid air turbine plant, moisture content in combustion air is increased in a gas turbine combustor, so that a fuel is deprived of combustion heat by the moisture content to cause decrease in flame temperature to result in reduction in generation of NOx. Also, since a turbine working fluid is increased in flow by addition of moisture, decrease in flame temperature is caused and generation of NOx is reduced also when a fuel is decreased in order to maintain number of revolutions constant.
Further, since decrease in flame temperature upon burning in a gas turbine combustor leads to decrease in heating value recovered in a regenerator, decrease in combustion air temperature leads to decrease in flame temperature and reduction in generation of NOx.
In this manner, addition of moisture is started whereby (1) increase in moisture content, (2) reduction in fuel, and (3) decrease in air temperature proceed simultaneously to lead to decrease in flame temperature, so that generation of NOx is decreased but flame stability is worsened conversely.
Hereupon, when air distribution supplied to the gas turbine combustor is beforehand set taking account of moisture addition, it is possible to appropriately set an air flow supplied to a premixing unit or a coaxial jet unit at a head of the gas turbine combustor so that flame blow off is not generated under a high moisture content condition.
With a gas turbine combustor, in which air distribution supplied to the gas turbine combustor is set in this manner, however, flame temperature is increased in a converse manner as described above before starting of moisture addition, so that there is a tendency that generation of NOx is increased while flame stability is ensured.
That is, in an advanced humid air turbine, a large change in a condition for generation of NOx and flame stability in a gas turbine combustor occurs before and after starting of moisture addition. Also, it is thought that after starting of moisture addition, valve control, a volume of an associated system, or the like at the time of increase in gas turbine loading brings about lag until moisture is actually added to air for combustion.
Also, it is thought that at the time of decrease in gas turbine loading, lag is brought about for the same reason until an air for combustion is decreased in humidity. For such change in a condition, there is also demanded control means for stable combustion of a gas turbine combustor with low NOx.
Hereupon, as disclosed in JP-A-2008-175098, combustion stability after humidification can be ensured by forming a part of combustion units of a gas turbine combustor, which comprises a plurality of combustion units supplied individually with a fuel, from those combustion units (combustion units provided with air nozzles, which impart swirl to air flow), which are more excellent in flame stabilizing performance than the other combustion units, and setting a large ratio of a fuel supplied to the combustion units of excellent flame stabilizing performance so that for a predetermined period of time after starting of humidification, combustion gas temperature of the combustion units of excellent flame stabilizing performance become above combustion gas temperature before starting of humidification, to thereby control a fuel.
In case of ensuring combustion stability against such change in humidity by applying the technology of the control device as disclosed in JP-A-07-189743, it is conceivable to measure the moisture content in compressed air and to control a fuel flow ratio on the basis of the measured value.
Also, it has been assumed by test data analysis thereafter that a change in fuel ratio, which is caused with the lapse of time, is attributable to gradual increase in moisture content of compressed air in starting of humidification. That is, it has been found that even when a humidification tower feed water volume is held under a particular condition in a state of humidification, increase in moisture content of compressed air involves a certain lag and the lag has a time span and a changed width, which affect combustion stability and a low NOx performance in a gas turbine combustor.
In case of by ensuring combustion stability against change in compressed air humidity by applying the technology of the control device disclosed in JP-A-07-189743, it is conceivable to measure the moisture content in compressed air and to control a fuel flow ratio on the basis of the measured value.
However, air at a humidification tower outlet is in a saturated condition and at high temperature around 100° C., and if a humidity sensor is mounted at the humidification tower outlet, there is possibility that error becomes large in humidity measurement with the humidity sensor. Also, air at a regenerator outlet is not in a saturated condition but is at high temperature of 450° C. to 650° C., and if a humidity sensor is mounted at the regenerator outlet, there is caused a problem that a high thermal resistance is demanded of the humidity sensor at the regenerator outlet.
Hereupon, in an advanced humid air turbine plant, there is demanded control means of a gas turbine combustor, which makes low NOx and flame stability compatible against that change in combustion air humidity, which generates a large change in condition after starting of moisture addition with respect to generation of NOx and combustion stability in the combustor.
It is an object of the invention to provide a fuel control method and a fuel control device of a gas turbine combustor for advanced humid air turbines, in which method and device the gas turbine combustor can perform combustion stably with low NOx in the case where a transient change in a condition against generation of NOx and flame stability in the gas turbine combustor occurs after addition of moisture to a humidification tower of an advanced humid air turbine is started.